1. Field of the Invention
The present invention relates to a semiconductor optical device, such as a semiconductor optical amplifier.
2. Description of Related Background Art
Generally, a semiconductor optical amplifier or amplifying apparatus is provided with a semiconductor laser structure which includes an active layer, a clad layer and the like. The semiconductor optical amplifier performs optical amplification for input light from outside, under bias current below its threshold injected thereinto. In optical communications, the optical amplifier has been developed as a device for compensating for optical loss which occurs in an optical fiber, at connections between fibers, etc.
However, when such semiconductor optical amplifiers are used in optical fiber communication systems, there exists the problem of polarization dependency of an optical amplification factor (i.e., amplification factors differ for different polarization modes). In general, the polarization state of output light transmitted through the optical fiber fluctuates with time. As a result, if such output light is supplied to the above-mentioned optical amplifier with polarization dependency, the level of output light from the optical amplifier will be unstable and fluctuate with time. Therefore, a receiver system is required to have a wide dynamic range, and various burdens are imposed on the receiver system. This is a fatal drawback that limits the scale of optical systems or networks.
Therefore, various efforts have been proposed for constructing a polarization insensitive optical amplifier (i.e., amplification factors for different polarization modes are substantially equal to each other). Among them, there has been proposed a method of using a strained quantum well structure for solving the polarization dependency of optical gain in its active layer. The strained quantum well structure has been used for controlling the oscillation wavelength and reducing the oscillation gain threshold in the field of semiconductor lasers as well as in the field of amplifiers, and thus the strained quantum well structure is a very useful technique.
Generally, the gain for transverse magnetic (TM) light is made equal to or larger than the gain for transverse electric (TE) light to achieve a polarization insensitive optical amplifier by using such a strained quantum well structure. In more detail, the degeneracy in a valence band is released by the effect of strain, and band structures of heavy and light holes are respectively shifted to make the transition energy between an electron ground level in a conduction band and a level of heavy holes (HH) in the valence band approximately equal to, or slightly larger than the transition energy between an electron ground level in the conduction band and a level of light holes (LH) in the valence band. When there is no other gain polarization dependency than the optical gain polarization dependency, those transition energies are made equal to each other. When there are other gain polarization dependencies than the optical gain polarization dependency, the latter transition energy is made smaller than the former. The light confinement for TE light is generally greater than that for TM light, so that the latter transition energy is made smaller than the former.
Several methods have been proposed for imparting strain to a quantum well structure so that the above-discussed desired energy levels can be obtained.
First, Japanese Patent Laid-Open Application No. 1-251685 discloses a method of imparting biaxial tensile strain to a well layer. The biaxial tensile strain is imparted to a second semiconductor layer by layering the second semiconductor layer on a first reference semiconductor layer (a substrate or a clad layer). The lattice constant of the second semiconductor layer is smaller than that of the first semiconductor layer. Since the biaxial tensile strain is imparted to the well layer, the band edge of light holes in the valence band is shifted towards a direction to narrow the band gap in the second semiconductor layer. As a result, the energy level of light holes in the valence band is lowered and approaches the energy level of heavy holes. Thus, desired levels can be obtained.
Second, Japanese Patent Laid-Open Application No. 4-27183 discloses a method of imparting biaxial tensile strain to a barrier layer. In a manner similar to the first prior art method, the band edge of light holes in the valence band of the barrier layer is shifted due to the effect of strain. As a result, a well of light holes becomes shallow and its energy level shifts. Thus, desired levels can be obtained.
Third, Japanese Patent Laid-Open Application No. 1-257386 discloses a method of forming an active layer that includes two kinds of well layers: a strained (biaxial tensile strain) well layer and a non-strained well layer.
Various problems, however, exist in those prior art methods. In the first prior art method in which the biaxial tensile strain is imparted to the well layer, though a large amount of energy shift can be obtained by a small amount of strain, the band gap of the well varies, and hence the wavelength, at which gain is obtained, also varies. In the second prior art method in which the biaxial tensile strain is imparted to the barrier layer, the wavelength, at which gain is obtained, remains almost unchanged, contrary to the first prior art method. However, a larger amount of strain is needed than the first prior art method to obtain a desired effect. Further, in the first and-second prior art methods, levels of HH and LH in one well are brought close to each other, so that the band mixing between HH and LH occurs. Thus, effective masses of the respective holes are greatly different from each other. As a result, a large number of carriers need to be injected into one well to obtain a large gain from an optical amplifier. Hence, unfavorable degradation of characteristics is likely to occur because of Joule heat generation and the like by high-density carrier injection.
Compared with those prior art methods, in the third prior art method in which the strained well is combined with the non-strained well, ground levels of HH and LH are respectively provided in the two wells, so that the band mixing effect in one well is reduced. Thus, the degradation of characteristics due to high-density carrier injection can be prevented. However, it is difficult to match the transition wavelengths of HH and LH with each other because of energy shift in the conduction band by strain.